Since the discovery in the mid 1980s of the perovskite family of HTS materials, extensive strides have been made in the ability to deposit high quality HTS films. Thin films formed from HTS materials are highly desirable for a variety of superconductive electronics applications including, for example, detectors, digital circuits, and passive microwave devices (e.g., HTS-based filters).
Over the years, several techniques have been developed for the deposition of thin films of HTS oxide materials. These techniques include sputtering, pulsed laser deposition (PLD), and metal-organic chemical vapor deposition (MOCVD). Illustrative HTS oxide materials include Yttrium Barium Copper Oxide (YBCO), Bismuth Strontium Calcium Copper Oxide (BSCCO), Thallium Barium Calcium Copper Oxide (TBCCO), and Mercury Barium Calcium Copper Oxide (HBCCO). Of these materials, YBCO is currently the favored compound for many applications due to YBCO's relatively smaller conduction anisotropy, high superconductive critical current in a magnetic field, and good chemical stability. In addition, as compared to the other HTS compounds, the relative ease with which high quality, single phase thin films of YBCO may be grown is perhaps its greatest attribute.
Nonetheless, thin film growth of these materials has still been difficult. In order to obtain high quality films, oriented, epitaxial growth (in-plane and out-of-plane) is necessary, meaning that the films can be grown only at high temperatures, typically above 700° C. Therefore, growth is only possible on a handful of single crystal substrates that satisfy strict requirements of chemical compatibility, lattice constant match, and thermal expansion match. In addition, the properties of the substrate must be suitable for the required application. For example, MgO is a substrate that meets the growth needs and also has sufficiently low loss for microwave applications.
Growth of HTS materials is further complicated by the fact that these compounds typically comprise at least three metallic species which are in oxide form. The growth methods employed must therefore be strictly controlled in order to achieve the proper film stoichiometry and uniformity. Furthermore, in-situ growth of these materials requires them to be oxygenated as they are grown, which is generally not compatible with many conventional techniques such as physical vapor deposition. Moreover, in certain applications, growth of HTS thin films on two sides of a single substrate is required.
Conventional in-situ growth techniques such as sputtering, pulsed laser deposition, and metal-organic chemical vapor deposition have all been successfully used for the growth of HTS thin films. There are, however, serious limitations to these methods including slow growth rate (sputtering), difficulty in controlling composition, poor reproducibility, poor film uniformity (sputtering, MOCVD), difficulty of achieving large-area deposition (PLD), and difficulty of scalability (all aforementioned techniques).
More recently, the technique of reactive coevaporation using a rotating oxygen-pocket heater has been used which addresses many of the limitations discussed above. In this technique, the substrates are held by gravity on a rotating substrate support member or turntable. The substrate support member containing the substrates is surrounded on the top, bottom, and sides by a cylindrically-shaped heater (i.e., a heater body) that radiatively heats the substrates to a uniform high temperature necessary for reaction. The substrate support member is rotatable inside the heater body and, during a portion of the rotation, is exposed to a vacuum chamber via a window disposed in the underside portion of the heater body. The vacuum chamber surrounds the heater body and contains the deposition sources for the reaction.
In one embodiment, the heater body also includes an oxygen pocket region located in the bottom portion of the heater body. Oxygen is fed into the oxygen pocket region and thereby exposes the substrates to oxygen during a portion of the rotation. A large pressure differential is created between the oxygen pocket region and the surrounding vacuum chamber. The pressure differential is maintained by a narrow gap formed between the rotating substrate support member and the oxygen pocket region, thereby resulting in a low rate of oxygen leakage from the oxygen pocket.
During the part of the rotation where the substrates are exposed to the vacuum chamber, the thin film constituents (typically metallic species) may be deposited onto the underside of the substrates using typical PVD techniques such as evaporation. Oxidation reactions take place when the substrates are rotated on the substrate support member into the oxygen pocket.
Reactive coevaporation using a rotating oxygen pocket heater has several advantages including, for example, the ability to evaporate the metallic species in vacuum without complications that would arise under the high oxygen pressure conditions needed for growth of HTS films. In addition, the rotating oxygen pocket heater technique permits deposition on substrates having a relatively large surface area. Moreover, this technique has the ability to deposit HTS materials on multiple substrates at once. Because the heater approximates a blackbody radiator, different substrate materials can also be incorporated simultaneously even if they have different absortivities. Finally, reactive coevaporation using the rotating oxygen pocket heater allows one to deposit HTS materials on both sides of the substrate sequentially because neither side of the substrate is in direct contact with the heater body.
While reactive coevaporation methods are well suited for the formation of HTS thin films, there remains a need to increase the capacity and stability of such methods so that reactive coevaporation can be implemented into a commercially viable manufacturing process. There thus is a need to increase the throughput and reliability of reactive coevaporation deposition systems and methods.